1. Field of the Invention
The present invention relates to random butadiene-isoprene copolymers with a trans-1,4 structure and the process for their preparation.
2. Description of the Invention
1,4-trans polybutadiene can be prepared either by using catalysts based on transition metals or with catalytic systems based on metals of group I and II. The transition metals include titanium, vanadium, chromium, rhodium, iridium, cobalt and nickel.
The 1,4-trans polybutadiene obtained with catalysis based on transition metals is a material with resin characteristics having two melting points of 50° C. and 150° C.
Catalysts based on vanadium are by far the most important for the preparation of high molecular weight polybutadiene with a high 1,4-trans content. The AlEt3/VCl3 system gives rise to the formation of polymers whose content of 1,4-trans unit is at least 99%.
A polybutadiene with a high 1,4-trans content can be obtained using soluble vanadium catalysts. AlEt3, VCl3×3 THF and AlEt2Cl—V(acac)3 systems used in benzene or in toluene provide a polymer containing over 99% of 1,4-trans units. Soluble vanadium-based catalysts are used at temperatures lower than 20° C. as their activity deteriorates considerably at higher temperatures. The use of chlorine donors however (for example CCl3COOH) which re-oxidize V(II) to V(III), also allow them to be used at 80° C.
Butadiene-isoprene copolymers with a 1,4 trans structure are described in U.S. Pat. No. 5,844,044. These are used as elastomeric components mixed with other elastomers for the preparation of tyres. They have the disadvantage however of not being completely random, as they have softening temperatures (see Tables 1 and 3 of U.S. Pat. No. 5,844,044). The structure of these copolymers, moreover, is not totally 1,4-trans.
Butadiene-isoprene copolymers with a 1,4-trans structure have now been found, which are completely random and therefore do not have the disadvantages described above.